The invention relates to a method and arrangement for measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. In the arrangement, one surface of the target to be measured is illuminated by an optical power source, and the illuminated area on the opposite side of the illuminated surface comprises at least one backing with known optical properties. A detector detects the illuminated surface in the area of the backing, and at least one optical property of the target to be measured is determined by means of spectral information obtained at the backing.